Currently, in a test technique for an image sensor, a test light is usually projected onto the image sensor. Sensed image data is transmitted by the image sensor to a processor. The image sensor is calibrated based on the test light conditions. If there is an error in the test light source, the image quality of the sensor is greatly affected. Therefore, the test light source characteristics must be very stable. The test light optical spectrum should not change with the driving current and voltage. The output intensity should not change with the temperature. The operation cycles of the light source should be more than 10K hours.
However, the light source for use in emitting the test light, its optical properties may be influenced by external driving circuits so as to produce flickers for the test light, darkness, or generated too much heat for the light source. For instance, a Cold Cathode Fluorescent Light (CCFL) can be taken into the light source for a thin-film transistor liquid-crystal display (TFT LCD). However, CCFL is a temperature sensitive component and the brightness changed with the ambient temperature. The second instance, the brightness of a fluorescent light may dim in cold weather. Furthermore the fluorescent light has flicker noise. The third instance, a calibrated incandescent or halogen light generates excessive heat and its lifetime is shorter. A calibrated incandescent light source has a typical of 100 operating hours. In addition, the incandescent light is multidirectional radiators. Thus, reflectors are needed. All the aforesaid light sources are not suitable for the test technique of the sensor in the mass production environment.
Moreover, White Light Emitting Diode (WLED) with serial connection has longer lifetimes, stable brightness and its radiation angle is less than 30°. Therefore, the WLED is a better choice for the light source of testing a sensor. However, the WLED with serial connection must serial connect seven-WLED in order to satisfy the test requirement for the brightness of the test light. The serial seven-WLED needs 22˜24V forward voltages. If a direct current/direct current booster is used to adjust the forward voltages to 22˜24V, the wave form of voltages may produce ripples and the stability of the brightness of the test light is also influenced.
Therefore, to enable the light source to emit a stable test light for ensuring the accuracy of the test outcome, the inventor of the present invention based on years of experience on related optical research invents a light source of testing a sensor, a test apparatus and a method to overcome the foregoing shortcomings of the prior arts.